This invention relates to steel furnaces of the form referred to by the term xe2x80x9ctap-off converterxe2x80x9d, namely a convertor which can be tilted so that molten metal therein is able to be drawn off through a tap hole.
The operator needs to know when slag is about to flow, or has started to flow, through the tap hole so that pouring of the steel can be discontinued. A conventional method of doing this employs a plug member (commonly called a xe2x80x9cdartxe2x80x9d) whose specific gravity is less than that of the steel, but greater than that of the slag, so that the dart tends to xe2x80x9cfloatxe2x80x9d at the interface between the molten steel and slag. The dart has a lower stem, which locates freely into the tap hole. The enlarged head of the dart, when it eventually lowers into position, will then close off the tap hole, at least partially, as the slag layer approaches the tap hole. What happens, in effect, is that the flow of material through the tap hole is substantially reduced as the dart head covers over or enters the tap hole. This enables the operator to allow most of the residual pool of molten steel to run off until such time as the slag itself starts to run off, whereupon the convertor can be tipped back again to retain the rest of the slag.
However, due to the vortex that is formed as the steel is discharging from the furnace, slag can still be drawn through the tap hole from above the steel still present in the furnace. We have discovered that if a reactive material such as carbonaceous material (e.g. wood, cardboard) or a reactive metal (e.g. aluminium), or a combination of these materials, is attached to or integrated with the dart head or stem, gases are created such that a bubbling reaction or turbulence will be set up around the dart and tap hole. This will act to disperse slag from around the area of the tap hole. The reaction serves to reduce the possibility of slag being drawn into the vortex and hence passing into the ladle below. As the purpose or the slag dart is to minimise slag that passes through the tap hole onto the ladle, this development will serve to enhance the effectiveness of the dart in performing its intended use.
According to the present invention, there is provided a tap hole dart for a steel furnace, having an enlarged head from which projects downwardly a stem which is designed to locate in the tap hole, the upper and/or lower face of the dart head and/or the stem being provided with a layer or sleeve of reactive material in the form of carbonaceous material or a reactive metal or a combination thereof.
The material which will cause the reaction could be added to the head or stem during manufacture, thus integrating the reactivity into the head and stem.
The term xe2x80x9creactive metalxe2x80x9d defines a metal which will vaporise or react with the steel at molten steel temperatures to create bubbling or turbulence of the liquid steel, without creating unacceptable contamination of the steel. Suitable metals for this purpose are aluminium and magnesium. Preferred carbonaceous materials are paper, cardboard or wood.